Ever since Stephen Paddock opened fire on 22,000 people at a country music festival last Sunday, investigators have been trying to piece together a profile of the killer. They may finally have a possible motive: Paddock’s mental health.

What do investigators think?

Paddock was an avid gambler and traveler who didn’t have big political or religious affiliations, police say. He was a very financially successful man, but police interviews have discovered that Paddock may have had a severe mental illness that went undiagnosed, according to ABC News.

People who knew or interacted with Paddock say he had a difficulty interacting with people, and was a “standoff-ish, disconnected man who had difficulty establishing and maintaining meaningful relationships,” ABC reported.

One law enforcement described Paddock as having social traits similar to other mass shooters, according to Fox News. Plus, given that those close to Paddock are surprised by his actions, an untreated mental illness could explain how he carefully planned the crime while maintaining his normal life.

What does Paddock’s girlfriend say?

Despite claims by those acquainted with Paddock, the mass killer’s live-in girlfriend, Marilou Danley, said there was no indication that Paddock would become violent. Paddock’s brother, Eric, was also surprised by his brother’s actions.

What do others say?

It was revealed late last week that the mystery woman spotted with Paddock just days before the crime was a Las Vegas escort. According to the Daily Mail, one prostitute who had frequent encounters with Paddock said he bragged about being “born bad” and said Paddock often liked to act out “violent rape fantasies.”

What does the lack of information suggest?

It’s so unusual after such a massive crime to not know the perpetrator’s motive, but that’s the case with the Vegas massacre. Investigators say they’ve “looked at literally everything,” but they still have little to go on.

What investigators do know is that Paddock spent months, maybe even a year or more, planning out his attack. So now, in addition to asking “why” Paddock committed the crime, investigators need to ask, “why so little motive?”

However, with more knowledge of Paddock’s mental state, police will likely be able to better understand what led him to commit the crime.